Medical electrodes have, in the past, taken many shapes and forms. Principally, they have been shaped according to the use for which they are intended. Electrodes used with monitoring apparatus, such as EKG and EEG machines, commonly have small round contact surfaces, whereas electrodes used with such stimulation apparatus as pain control devices tend to be larger and have rectangularly and other conveniently shaped contact surfaces. Whether intended for monitoring or stimulation use, a design objective for each electrode group has been, and continues to be, good electrical signal transmission between a patient's skin surface and the electrical cables connected to a particular piece of apparatus. With respect to stimulation and monitoring electrodes, efficient signal transmission across the epidermis conductor interface is desirable. Further, with respect to stimulation electrodes, effective signal transmission free of current concentration points or “hot spots” is also desirable.
Of the electrodes presently available, many offer combination structures including a metallic or otherwise conductive support member to which an electrical wire from an associated apparatus may be attached.
Certain of the currently available electrodes, including electrical stimulation electrodes are disclosed in U.S. Pat. Nos. 4,722,354; 4,736,752; 4,819,328; 5,038,796 and 5,450,845 to Axelgaard et al which are hereby incorporated by reference to show various electrode designs including but not limited to medical electrode shapes, structures, materials and methods for connecting said medical electrodes to the appropriate electrical apparatus.
In many instances, the medical electrodes of the prior art need the addition of generous amounts of an electrode paste or gel applied directly to the conductive support member to enhance conductivity across the skin-electrode interface to the point where acceptable operating conditions are achieved.
Other prior art electrodes teach the incorporation of an open cellular skin interface pad secured to a conductive support member. This pad, as shown in U.S. Pat. No. 3,817,252, is often sponge material which functions to hold an amount of electrolyte solution in order to enhance conductivity across the skin-pad interface. Such an interface pad can be, alternately, saturated with electrode pastes or gels of the type that do not run or evaporate as readily as electrolyte solutions.
The prior art elect-codes that require an electrode paste or gel or electrolyte solution provide a structure which does not always maintain constant, efficient and effective electrical transmission for long periods of time without the need for additional electrode paste, gel or solution. Moreover, there is a tendency while using these electrodes, for the electrode gel to separate and/or to flow to a nonuniform thickness. Under these conditions, sections of the conductive support member could be exposed to the skin and local hot spots can result which can cause discomfort if not severe enough to cause burns to the patient's skin. Therefore, medical electrodes wherein the adhesive, itself, provides the conductive interface between the skin and the electrical connector are very desirable. An electrode of this type is disclosed in U.S. Pat. No. 4,066,078 to Berg. In this patent, the polymer itself acts as the adhesive and, through the quaternary groups attached to the polymer backbone, provides a conductive interface.
Nevertheless, others have continued to formulate adhesive materials that effectively adhere to the skin and the materials that can be utilized in fabricating a medical electrode and also provide adequate conductivity. See, for example, U.S. Pat. Nos. 4,830,776; 4,274,420; 4,777,954; 4,699,146; 4,158,696; 5,024,227; 4,243,051; 6,115,625, etc., which exemplify the continuing search for conductive adhesive materials for use in medical electrodes.
Acrylic hydrogels are commonly used biomedical hydrogels due to low cost, absorbency, and ease of fabrication. These hydrogels may be used as contact lens materials as disclosed in U.S. Pat. No. 5,712,356 and as conductive electrode adhesives as disclosed in U.S. Pat. No. 5,868,136.
Vinyl ether monomers are biocompatible and environmentally friendly, and glycol vinyl ether monomers are water-soluble. Vinyl ether homopolymers are also flexible and elastic. The only commercial method of vinyl ether homopolymerization is cationic and can not be done in the presence of water.
It has now been found that the free radical copolymerization of glycol vinyl ether monomers with acrylic monomers in water promotes a high degree of acrylic polymerization and imparts greater flexibility and elasticity to the resulting acrylic copolymer than other co-monomers. Moreover, as compared to acrylic hydrogels without copolymerized glycol vinyl ether, copolymers of acrylic acid and a glycol vinyl ether provide films that are softer, has less aggressive adherence to skin and separate from skin without stringing or leaving residue on the skin.
However, a major problem that had to be overcome was that water-soluble acrylic monomers hydrolyze glycol vinyl ether monomers at the optimum conditions for free radical polymerization. A by-product of this hydrolysis is an irritating aldehyde. This problem has been solved by the present invention by reacting these aldehydes into biologically benign compounds. The amount of irritating residual acrylic monomers is also reduced by the high degree-of-polymerization of the electrode adhesives of this invention.
An objective of this invention, therefore, is to provide an electrode with an improved electroconductive skin-interface substrate, which will perform a similar function to, and eliminate the need for, an electrolyte solution, electrode paste or electrode gel.
Another objective of this invention is to provide an electrode with a skin-interface substrate having pressure sensitive adhesive properties which will enable the electrode to adhere to the skin without the use of tape or other securing mediums.
Another objective of this invention is to provide an adhesive substrate that has high tack so that minimal pressure is needed to apply it to the skin but is such that it can also be easily separable from the skin upon removal without any noticeable residue.
A further objective is to provide an electrode with a non-liquid skin-interface which is a film which will maintain a uniform thickness and will not separate to expose sections of a conductive support member to the skin.
Yet another object is to provide an electrode with an electroconductive skin-interface that will not dry out or decomposed for period of years even if exposed to the atmosphere.
An even further objective is to provide an electrode having a skin-interface substrate which will not decompose or dry out like electrodes utilizing electrolyte solutions under long periods of use.
Other objects and advantages of the present invention will become apparent from a careful reading of the specification below.